1. Field of the Invention
The invention relates to a device, or system, for performing transmission measurements with the aid of microwaves for the continuous determination of the concentration of various types of solid or liquid substances including, but not limited to, syrup, or sugar juice, fertilizers, and fluids found in sewage treatment plants, milk processing and the paper industry, the device having two components which are introduced into the material to be measured and operate as a transmission and receiving antenna, respectively. For example, measurements of the concentration of sugar juice may be determined during a boiling step performed in a boiling apparatus in the course of a sugar crystallization process,
2. Prior Art
For many years microwave measuring processes have been regularly used in connection with the control of industrial processes. Typical applications include, for example, the determination of moisture in bulk materials and control of the fill level in large tanks and containers. Microwave measurement is also used for determining the density and concentration of solid and liquid materials to be measured. The transmission and reception of the microwaves takes place by means of so-called applicators, which are essentially used in connection with four methods: transmission methods, reflection methods, measuring of samples in a resonator, and the placement of the material to be measured as the terminal of a line. While the first two methods are mainly employed for on-line measurements, the last two are used for the testing of individual samples.
The determination of physical parameters, such as the density or concentration of materials to be measured in closed metal containers, for example, basically requires the insertion of the applicator into the container, for example by means of a flanged connection, so that a direct interaction of the electromagnetic waves with the material to be measured can take place. Furthermore, during on-line operation the determination of the measured value requires the continuous detection of a representative sample of the material to be measured by the microwaves.
The employment of microwave measuring methods for the determination of physical parameters, such as density, concentration, or moisture content of a material to be measured, has several advantages over conventional measuring methods, such as measurements of conductivity or capacitance. With increasing frequencies above 1 GHz, the interfering effects of, for example, ion conductivity in the measuring sample, or contacting problems, become more and more negligible. It is possible by means of modem strip conductor circuits to achieve very compact microwave modules, for example synthesizers, modulators or demodulators, with a large dynamic range in respect to attenuation of up to 80 dB, and phase-shift resolutions of up to a tenth of a degree.
If a dielectric is located in an alternating field, the dipoles can follow the field at sufficiently low frequencies. The electrical field and the polarization of the medium are in phase. If, with a further increase of the frequency, the dipoles can no longer follow the outer field, a phase shift between the polarization and the electrical field occurs. Losses occur because of this, which lead to heating of the sample, and the value of the dielectric constant (DC) becomes smaller (dispersion). A description of the relationship between the electric alternating field E and the polarization P is made possible by introducing the complex dielectric constant: EQU .di-elect cons.=.di-elect cons.'-j.di-elect cons.".
Here, .di-elect cons.' describes the proportion of P, which is in phase with field E and which for the frequency f=0 makes a transition into the static dielectric constant. .di-elect cons." describes the proportion of P which lags behind field E by 90.degree., and is therefore a measure for the dielectric losses.
This explanation suggests that in the microwave range a determination of the concentration can be based on the measurement of the real portion, .di-elect cons.', as well as the imaginary portion, .di-elect cons.", of the complex DC, or respectively of values, which are linked with these. Since the complex DC greatly depends on the physical properties of the material to be measured, for example the concentration, and on the selected high frequency, a frequency should be selected wherein the effect of the concentration to be measured on the resulting complex DC of the sample is as large as possible. To increase the measuring accuracy, the observed measured values can also be averaged over a larger frequency range.
A procedure for determining complex dielectric constant is described in Klein, A., "Ein Verfahren zur schnellen Bestimmung der nichtmagnetischer Materialien im Mikrowellenbereich" (A method for the rapid determination of the complex dielectric constant on non-magnetic materials in the microwave range), ARCHIV FUR ELEKTRONIK UND UBERTRAGUNGSTECHNIK (Archive of electronics and transmission technology), Vol. 31 (1977) pp501-504.
Since two measurements, which are independent of each other, are required for determining .di-elect cons.' and .di-elect cons.", a complex transmission factor t is defined, which can be split into the magnitude T and the phase B. Here, t is defined as the ratio of the electric field strength of the wave E.sub.t transmitted through the sample and the electric field strength of the wave E.sub.j incident on the sample, as follows: EQU t=T.multidot.e.sup.jB =E.sub.t /E.sub.j
When determining the complex DC .di-elect cons. from T and B it must be assured that the electromagnetic wave is sufficiently attenuated by the material to be measured, so that the reflections at the boundary transitions can become negligible.
The measuring process employed in the operation of a system according to the present invention is described in European Patent No. EP 0 515 831 A3, German Patent No. DE 34 25961 A1 and published UK Patent Application GB 2147110A, published May 1, 1985.
A device in accordance with the described type of transmission measurement with the aid of microwaves is disclosed in German Utility Model DE 296 17 488 U1, where two antennas, which are directly dipped into the medium to be measured are used with this "submerged sensor". Here, one antenna acts as the transmitting component, the other as the receiving component. Both antennas are designed as monopole radiators and are positioned at a suitable measuring location inside a boiling apparatus with the aid of an immersion tube. The control of the boiling process in evaporation crystallizers in the sugar industry is cited as a typical application of this solution. To this end, the antennas are installed in a boiling apparatus, and the concentration of the sugar syrup can be continuously determined during the progression of the boiling process by performing transmission measurements, such as has been basically described above.
Because of its specific structural characteristics, this already known solution has some serious disadvantages, notably in that a correct and precise measurement can be made only if definite marginal conditions and parameters are maintained.
The monopole radiators used in connection with the already known utility model have a very narrow bandwidth, which reduces the measuring accuracy since, for determining the measuring values for the phase shift and attenuation, it is not possible to measure over a larger frequency range (sweeping), which would increase accuracy.
Performing the measurement with the already known utility model with the two monopole antennas is essentially based on back-scattering of the microwaves because of the spherical characteristics of the radiation field of these monopole antennas. This results in very high intrinsic attenuation (attenuation without material to be measured), which limits the dynamic measuring range of the arrangement.
Positioning of the monopole antennas in the material to be measured (sugar juice/syrup) with the aid of the immersion tube requires the supply of high frequency energy by means of a coaxial line inside the immersion tube from the outer wall of the boiling apparatus to the two monopole antennas. The temperature dependency of these coaxial cables results in a considerable disadvantage, in that the temperature-dependent calibration of the arrangement must be strictly defined and monitored because of the large temperature variations in the boiling apparatus (approximately 75.degree. C. during boiling and approximately 125.degree. C. during evaporation). Thus, a relatively large expenditure is required here in order to prevent distortion of the results of the measurements.